Data migration for Publication first_published_at
Update Publication to use first_pubished_at

This drops use of redundant publication_date attribute. We still have
to actually drop the column from the database, but that should happen
after this code is deployed.
Make tests more robust
Merge pull request #800 from alphagov/55389442-fix-publication-dating

Fix publication dating (55389442)

https://www.pivotaltracker.com/story/show/55389442